Histrelin, a GnRH analogue, is being studied for prevention of cyclic attacks of the acute intermittent porphyria, hereditary coproporphyria and variegate porphyria. These inherited (autosomal dominant) metabolic diseases are manifest most commonly in women during their reproductive years. Cyclic attacks occur frequently in some women. Up to twenty patients will be treated with histrelin. Observations include frequency of attacks, urinary porphyrin precursors, and plasma hormone levels. This project is likely to demonstrate safety and efficacy of histrelin for this indication and provide a better understanding of the natural history of cyclic attacks of porphyria.